The DEA recently decided to place the drug kratom into schedule 1 of the Controlled Substances Act, the most restrictive regulatory category, "to avoid an imminent hazard to the public safety." The ban is likely to go into effect on Friday.

"The DEA’s decision to place kratom as a Schedule I substance will put a halt on federally funded research and innovation surrounding the treatment of individuals suffering from opioid and other addictions — a significant public health threat," the letter states. Mitigating the effects of the national opioid epidemic -- which along with heroin killed close to 30,000 people in 2014 -- has been a major goal of the Obama administration's drug policy.

Noting that the DEA did not take public comment from "consumers, researchers, and other stakeholders" before the decision, the representatives say that the DEA's move "threatens the transparency of the scheduling process and its responsiveness to the input of both citizens and the scientific community."

The letter notes that federal research grants from the National Institutes of Health and the National Institute on Drug Abuse, led to a patent application for the use of kratom in "treating withdrawal from addictive compounds." The researchers involved with that patent said that the active chemical in kratom "completely blocked all withdrawal symptoms and could provide a remarkable step-down-like treatment for people addicted to hardcore narcotics such as morphine, oxycodone or heroin" in a 2013 press release.

The White House's Office of National Drug Control Policy referred inquiries to the Drug Enforcement Administration. The DEA did not respond to a request for comment.

Kratom is a Southeast Asian plant that has shown promise as a potential replacement for highly addictive and deadly opiate painkillers. Anecdotal evidence suggests people can use the drug to wean themselves off opioids and alcohol, as well as to treat otherwise intractable chronic pain.

But the DEA has said that these opiate-like effects are sufficient grounds to temporarily place the drug into schedule 1. In a Federal Register notice, the DEA cited more than 600 poison control center calls related to the drug between 2010 and 2015. It also noted 15 deaths linked to kratom use between 2014 and 2016, although a DEA spokesman later clarified that in only one of those cases was kratom the only drug involved.

Scientists studying the drug's effects are worried that the DEA ban will stifle further research. Andrew Kruegel of Columbia University, who studies how compounds in kratom act on the brain's pain receptors, told Scientific American that he'll have to destroy all the kratom samples in his lab because he's not currently licensed to work with Schedule 1 substances.

In a conference call with reporters on Tuesday, Kruegel said that "the new regulations will restrict our ability to pursue the development of new painkillers" that could replace the deadly opiates currently causing thousands of deaths annually.

Kruegel's colleague, Susruta Majumdar of the Memorial Sloan-Kettering Cancer Center, said in the call that kratom research could lead to the "holy grail" of painkiller medicine -- a drug that's both effective at treating pain, and unlikely to be abused. But "a ban will definitely hinder our understanding" of how the compounds in kratom work.

Other researchers have echoed similar concerns. Marc Swogger, a clinical psychologist atthe University of Rochester Medical Center who has published research on kratom use, said in an interview earlier this month that "the research indicates that this is a pretty mild substance" and that "criminalizing kratom use is insane to me."